The present invention relates to a process for producing filters which contain as the filter material a roughened, cross-wound yarn of fine individual fibers. The invention is particularly applicable to production of soot filters formed of heat-resistant yarn. Soot filters of this type are preferably used in the exhaust gas stream of an internal combustion engine.
U.S. Pat. No. 4,324,572 discloses soot filters comprising roughened silicon dioxide fibers cross-wound onto supporting tubes. To produce the individual soot filter elements it is thus necessary to subject the corresponding fiber to a treatment which, on the one hand, produces an adequate filter performance and on the other makes possible inexpensive manufacture.
It is known in textile processing, for example from German Offenlegungsschrift No. 3,335,752, to produce yarn effects by roughening yarns with roughening rollers, but yarns processed in this way are not suitable for the significantly higher technical requirements of the filter industry. Consequently, existing yarn roughening processes from the textile field are not readily transferable to filter yarns. With the known textile yarn processes it is simply not possible to roughen any heat-resistant yarn in such a way that it exhibits a sufficient filtering action for a soot filter.